This invention relates to a coating applied to a polystyrene pattern for casting metal by a lost foam process. More particularly, this invention relates to a dual coating that initially has a low permeability to pattern decomposition vapors, but thereafter develops a high permeability, which variable permeability reduces turbulence in metal replacing the pattern, while ultimately exhausting substantially all vapors, to reduce gas entrapment in the metal that would form porosity in the casting.
In casting by a lost foam process, molten metal is poured into a polystyrene pattern embedded in a sand mold. The metal vaporizes the polystyrene and flows into the resulting void, thereby concurrently consuming and replacing the pattern to form a casting. A coating is preferably applied to the pattern prior to casting to thermally insulate the metal to prevent freezing before the entire pattern is replaced. The coating may also keep the mold from collapsing prior to being filled, particularly when casting relatively high melting metals, such as iron, that consume the pattern substantially in advance of the melt.
During casting, pattern decomposition vapors escape through the coating into the sand mold. Vapors that cannot escape collect in pockets in the metal that form undesirable pores in the casting. Thus, a coating having a high permeability is desired to exhaust substantially all vapors. Pores also form as a result of turbulence in the melt that entrap vapors. Severe turbulence may even entrap polystyrene before vaporization, whereupon even a small piece may produce a relatively large pore. The severity of turbulence is related to the rate at which metal fills the mold, faster filling producing generally greater turbulence. It has been found that vapor buildup in advance of the metal slows the flow and thus reduces turbulence. Suitable vapor buildup is produced by a coating having a low permeability. Thus, both low and high vapor permeability through the coating reduces porosity in the casting.
Therefore, it is an object of this invention to provide an improved thermally insulative coating applied to a vaporizable pattern for lost foam casting of metal, which coating is adapted to have initially a relatively low vapor permeability to cause pattern decomposition vapors to build up and slow melt flow to replace the pattern, thereby reducing vapor-entrapping turbulence in the melt. After a suitable delay, the coating develops a high vapor permeability to allow substantially all vapors to escape from becoming entrapped in the melt. Thus, the coating sequentially exhibits both low and high vapor permeability during casting at suitable times for reducing porosity in the product casting.